The bubble chamber

The bubble chamber is no longer in wide use for Particle Physics experiments, but the
photographs that these machines produced are excellent for 'seeing' particles and the way
that they behave. Therefore they form an excellent teaching tool, and use ideas - not
unlike the physics of the more familiar cloud chamber - which is easy to understand.
We think they are BRILLIANT!

The Bubble Chamber - the student-friendly
particle detector.

Following trails.

If two aeroplanes with vapor trails behind them were to approach each other, circle
around, and then go their separate ways, the fact that they had done so would be apparent
for quite a while. A permanent record of the encounter could be obtained by taking a
photograph of the vapor trails.

Detectors.

A particle detector is an instrument that can record the passage of particles through
it. From a teaching point of view, the bubble chamber is a particularly valuable detector
because it provides a real photograph of the trajectories of charged particles travelling
though it. The only processing involved is in the photographic dark room. The dark lines
on this photograph are an example.

There is an introduction to particle detectors, with particular reference to the bubble
chamber. This article contains a general introduction to bubble chambers (followed by a
careful derivation of the rate at which charged particles lose energy by ionization,
dE/dx; suitable for first-year undergraduates).

The Bubble Chamber.

The bubble chamber consists of a tank of unstable transparent liquid - superheated
hydrogen in our case. When a charged particle forces its way through the liquid, the
energy deposited initiates boiling along the path, leaving a trail of tiny bubbles.

The superheated liquid is prepared by starting with the very cold liquid under pressure
(about 5 atmospheres and 3K) and then, just before the particle beam arrives, the pressure
is reduced suddenly by expanding the volume by about 1% by means of a piston.

Gargamelle, showing the cylindrical chamber and the electromagnets

How long are the tracks?

After the particles have passed through the liquid, the bubbles are allowed to expand
until they are a few tenths of a millimeter across, big enough to be photographed by flash
illumination. To enable physicists to reconstruct the event in three dimensions,
photographs from more than one camera are needed.

The relativistic particles cross the few meters of liquid in a few nanoseconds (1ns =
10-9s); the growth time of the bubbles is about a
million times longer, about 10ms.

A simple calculation.

Assuming the particles are travelling at the speed of light (and most
are quite close to this) how far would they travel in 1 ns?

Taking the next picture.

Once the photographs are taken (more than one view is needed to reconstruct the events
in 3D), the bubbles are collapsed by recompressing the liquid, and the bubble chamber is
prepared for the next burst of particles. This is quite slow process - maybe a few seconds
- so the number of pictures is limited and the analysis of them is labor intensive. Modern
detectors allow many thousands of events per second, and the images are computer
generated; bubble chambers are no longer used.

Although extinct, bubble chamber pictures are remembered fondly, like dinosaurs. Unlike
dinosaurs they provide a direct way of ëseeingí events that are real today.

Why are bubble chamber pictures so useful?

The great advantage is their ability to pick up details of a complicated interaction,
and by following the trails of bubbles one can see subsequent interactions and decays of
the products of the initial interaction.

Furthermore the principles by which the pictures are made and analyzed are easily
understood.